Abstract 907: Identification of a stem cell-specific secretory immune regulatory ligand

Abstract Immunotherapies have transformed cancer treatment, but subsets of patients experience differential responses. Patients with squamous cell carcinomas (SCCs) in various tissues exhibit a high rate of relapse after their initial response to immunotherapies. Recent research highlighted the critical roles of a group of stem cell-like tumor-initiating cells (TICs) in driving the relapse of both cutaneous (CSCCs) and head and neck (HNSCCs). These TICs exhibit superior resistance to anti-tumor immunity despite their intact antigen presentation. Although TICs are the root cause of SCC relapse, they compose less than 5% of the total tumor cell population. Therefore, their unique immune resistance mechanisms have been largely overlooked. Here, we aim to identify the special immune regulatory mechanisms that are specifically activated in TICs endowing them with stronger immune resistance. By analyzing single-cell RNA-seq data profiling the gene signatures of various tumor populations in CSCCs, we identified cytotoxic T lymphocyte antigen 2a (Ctla2a) was specifically activated in TICs. Ctla2a is characterized as a cysteine peptidase inhibitor and has high homology to the I29 inhibitory domain in the proregion of mouse and human cathepsins. Silencing Ctla2a in SCC cells reduced tumor growth, enhanced CD8+ T cell infiltration, and elevated their antigen-specific cytotoxicity. Importantly, we have found that the conditioned medium collected from SCC cells overexpressing Ctla2a is sufficient to blunt the cytokine production of CD8 T cells, suggesting that Ctla2a proteins are secreted from TICs to modulate T cell immunity. Ongoing aims focus on 1) how the peptidase inhibitor activity of Ctla2a reduces cytotoxic cytokine production in CD8+ T cells and 2) whether the I29 inhibitory domain of human cathepsins is capable of conferring the immune-suppressive activity observed in Ctla2a expressing SCCs. Taken together, these results suggest that TICs can secrete these cathepsin inhibitory peptides to perform moonlighting functions and modulate CD8+ T cell anti-tumor cytotoxicity. This interaction may serve as a target for next-generation immunotherapy that is able to blunt TIC-specific immune resistance. Citation Format: Benjamin T. Nicholson, Ama Ofori, Jennifer Good, Weijie Guo, Yuxuan Miao. Identification of a stem cell-specific secretory immune regulatory ligand [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 907.